Infrared (IR) proximity sensors are becoming popular in cell-phone and handheld-device applications. For example, the sensor can be used to control a touch-screen interface for portable electronics devices. When an object, such as a person's finger, is approaching, the sensor detects the object. When the object is detected, a touch-screen interface or the like may perform an action such as enabling or disabling a display backlight, displaying a “virtual scroll wheel,” navigation pad or virtual keypad, etc.
A conventional analog-output IR proximity sensor typically includes discrete components, including an infrared (IR) light emitting diode (LED), a switch to turn the IR LED on and off, and an IR photodiode (PD). During normal operation, the switch delivers current to the IR LED. The IR light emitted from the IR LED (or at least a portion of the IR light) will be reflected by an object when there is any, and be received by the PD. The PD converts the reflected light, as well as ambient light, to a current going to a resistor connected in parallel with the photodiode. The analog output is the voltage across the resistor. The intensity of the reflected IR light received by the photodiode is decreased at a rate of about 1/(4*X^2), where X is the distance between the object and the PD. However, as just mentioned, the total IR light received by the PD also includes ambient IR light, which may be from sun light, halogen light, incandescent light, fluorescent light, etc. FIG. 1A shows the spectrum of these different types of light.
In order to improve the signal-to-noise ratio of the sensor, the PD of the convention analog-output proximity sensor is typically made with a relatively large sensor area and with a special package, which has a narrow band-pass filter with the peak at the IR LED's emitting wavelength. A typical spectral response of such an IR PD is shown in FIG. 1B. Additionally, to improve the signal-to-noise ratio, a relatively high current is typically used to drive the IR LED in order to emit a stronger IR light signal. The use of the large size sensor area, the special package and the high current make such conventional IR proximity sensors unsuitable, or at least not optimal, for cell-phone and other handheld-device applications.
Since proximity sensors are meant to operate in a user environment that includes ambient light, such sensors should preferably be able to detect weak signals (for lower power operation and/or for longer distance detection) even in the presence of strong ambient light. However, the photo current generated by ambient light in such sensors often overwhelm the sensors. This results in sensors that are prone to falsely trigger, or not trigger when the should, due to strong ambient light interference.
Some conventional techniques for attempting to reject ambient light use a transimpedance amplifier, as described with reference to FIGS. 1C and 1D. In FIG. 1C, a high-pass resistance-capacitance (RC) network is provided at an input of a preamplifier 102, to pass the high frequency components of a signal while blocking the low frequency components. However, this solution has two major drawbacks. First, a very large resistor (R) and a very large capacitor (C) are required to achieve the low cutoff frequency, which is undesirable because such passive components occupy a very large chip area and are sensitive to parasitic-coupled noise. Second, the voltage across the resistor (R) varies with the average photo current, which causes the preamplifier's common-input (and therefore the overall performance) to be directly affected by the ambient light level. In the alternative configuration shown in FIG. 1D, the RC network is replaced by an active feedback loop around a transimpedance amplifier. In this configuration, the ambient light rejection is achieved by using analog level detection based on the peaks of the photo current signal. However, this technique assumes that the average current is constant, requires a reset mechanism and has a very low speed operation.